The study published in PNAS looked at how speech can emit thousands of ‘oral fluid droplets’ every second and found that those droplets can remain suspended in the air, allowing them to transmit diseases especially in confined spaces.

Its conclusion “adds weight to the need for social distancing”, one expert has said, as well as raising important issues about the potential spread of coronavirus in confined spaces like offices and factories.

The researchers said: “Speech droplets generated by asymptomatic carriers of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) are increasingly considered to be a likely mode of disease transmission.

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“In a closed, stagnant air environment, they disappear from the window of view with time constants in the range of eight to 14 min, which corresponds to droplet nuclei of ca. 4 μm diameter, or 12- to 21-μm droplets prior to dehydration.

“These observations confirm that there is a substantial probability that normal speaking causes airborne virus transmission in confined environments.

“This direct visualisation demonstrates how normal speech generates airborne droplets that can remain suspended for tens of minutes or longer and are eminently capable of transmitting disease in confined spaces.”

The study found that the probability that speech droplets would pass on an infection would take into account how long that droplet remained airborne.

But it said that considering that frequent person-to-person transmission had been reported in community and health care settings, it appeared likely that it could be relevant for coronavirus and other contagious airborne respiratory diseases, such as influenza and measles.

Professor Lawrence Young, professor of molecular oncology at the University of Warwick, said: “This study measured the size and spread of oral fluid droplets using a laser.

“It convincingly shows that normal speech generates airborne droplets that can remain suspended in the air for tens of minutes or longer.

“This suggests that virus from an infected individual could be transmitted this way in confined spaces, however there is no direct analysis of the presence of viruses in the droplets or their ability to pass on infection.”

He said one of the main assumptions in the paper is that each virus particle in a droplet is equally capable of causing an infection – which isn’t known to necessarily be the case for COVID-19.

But he said: “It adds weight to the need for social distancing and raises important issues about the potential for the virus to spread in confined spaces such as offices and factories.

“It also highlights the problem of virus transmission from infected individuals who do not have symptoms.”